This invention relates to the clamping of concentric well casings, where an inner well casing is to be clamped in position relative to an outer well casing, to achieve a desired relative axial position between the casings, for operational reasons axial fixing between coaxial well casings over a range of positions may be required at various times during drilling and/or production from wells, and the present invention makes it possible to clamp one or more casings within another at any desired position and subsequently to unclamp the casings for disassembly, or to change their relative positions and then reclamp the casings in a new relative position. As the clamping mechanism is preinstalled and can be externally activated, the blowout preventers can remain in place throughout the installation, clamping or release of the subsequently installed casing.
In oil and gas wells, it is conventional to pass a number of concentric tubes or casings down the well. An outermost casing is fixed in the ground, and the inner casings are each supported in the wellhead or in the next outer casing by casing or tubing hangers.
These casing hangers may take the form of a body with interengaging internal shoulders on the outer casing and a body with external shoulders on the inner casing hangers, located at fixed positions on each previously installed casing.
There are however applications where a fixed position casing hanger is unsatisfactory, because the hang-off point of one casing on another may require to be adjustable.
The invention has particular application for such casing and tubing hangers, which require adjustment.
Where drilling or production wellheads have to accommodate a casing or tubing without predetermined hang-off point, it has been known to use casing slip-type support mechanisms.
It is also known from European patent number EP251595B2 to use an adjustable landing ring on a surface casing hanger to accommodate a space-out requirement.
It is furthermore known that where production wellheads have to accommodate casing or tubing with a tension load greater than the running weight, retractable shoulders or internal wedge mechanisms have been used to allow passing of the casing or tubing hanger, and re-tensioning to a predetermined point.
According to a first aspect of the present invention, there is provided a pre-installed clamping arrangement for clamping a subsequently installed tubular well casing of a first diameter within a previously installed tubular casing of larger internal diameter, the arrangement comprising a sleeve associated with the large diameter casing, the sleeve having a collar at one end which has an external tapered surface, the arrangement also including an annular component with an internal tapered surface, the sleeve and annular component being relatively axially moveable between a first position in which the tapered surface of the annular component exerts minimal or no radial force on the collar and a second position in which the tapered surface of the annular component exerts sufficient radial force to distort the collar into the bore of the larger diameter casing, to grip the well casing of smaller diameter, the arrangement also including a removable device for maintaining the surfaces in the first position, and separate means for urging the annular component axially against the collar.
The sleeve may be of one piece with the large diameter casing, but more probably will be a separate component which could either be threaded onto the casing or be located in a suitable locating and receiving area on the casing.
The clamping arrangement preferably also provides a sealing function across the interface between the tapered surfaces, either through the metal/metal contact between the tapered surfaces, or through a separate seal body. Where the sleeve is a separate component from the larger diameter casing, there may be a metal/metal seal between the tapered surfaces and, in addition, a separate seal between the sleeve and the casing.
The device for maintaining the surfaces in the first position is preferably a spacer ring. The spacer ring may be removable, or may be a ring which can be moved axially by rotating it on a thread.
The sleeve can be formed as part of a casing hanger used for supporting a casing in a well.
The annular component can be a wellhead spool, and means can be provided to move the annular component axially in a direction away from the sleeve. This means for moving can comprise a chamber between the sleeve and the annular component, and the chamber can be pressurized to urge the wellhead component away from the sleeve.
The means for urging the annular component axially against the collar can comprise radially extending bolts extending through threaded bores in the annular component and each ending in a tapered dog, and recesses around the larger diameter casing, the recesses having inclined flanks and being positioned so that when the bolts are screwed in, the dogs enter the recesses and make contact with the inclined flanks, and as the bolts are screwed further in, the annular component is drawn further towards the sleeve.
The internal bore of the larger diameter casing may have a constant internal diameter, and the sleeve can be located between the larger diameter casing and the annular component, and when the arrangement is in use, the sleeve is in abutment with the larger diameter casing.
According to a second aspect of the present invention, there is provided a pre-installed clamping arrangement for clamping a subsequently installed tubular well casing of a first diameter within a previously installed tubular casing of larger internal diameter, wherein the larger diameter casing has a wall thickness which is sufficiently thin to allow the casing wall to be distorted inwards to grip the smaller diameter casing, the arrangement also comprising a compression unit which includes a compression collar surrounding the larger diameter casing, a compression ring axially movable relative to the collar and means for producing relative axial movement between the ring and the collar, the compression ring and compression collar having oppositely directed axially tapered annular surfaces, so that relative axial movement between the collar and ring produces a reduction in the internal diameter of the unit to distort the larger diameter casing inwards to grip the smaller diameter casing.
The tubular annular walls of oil well casings have to withstand substantial pressures, and it is this requirement to withstand certain pressures which generally determines the wall thickness of the casings. In most cases, casing walls will be too thick to allow inward deflection to grip an internal component. However by making the walls thin enough to allow such deflection, it becomes possible to achieve the advantageous clamping arrangement of the invention. It will be a matter of trial and error, or of relatively straightforward calculation, to determine the appropriate casing wall thickness for any particular application. Factors which have to be taken into account are the gap between the larger and smaller diameter casings (this gap has to be bridged when the clamp is tightened), the overall diameter of the casings and the material of which they are made. It is desirable to maintain deflection of the casing wall in the elastic range, thereby allowing the casing to expand to it""s original diameter once the clamping or compression force has been removed. This allows the clamping arrangement to be reversed or disengaged relatively quickly and easily without any permanent alteration to the casing. The clamping system can, of course, plastically deflect the casing should such be required by a particular application. One such application would be clamping more than one inner casing. In such a case, it is likely that the outer casing would be plastically deformed because of the greater clamping force required to adequately grip the most inner casing.
The casing may be divided axially into different sections, and it can then be appropriate to make the section of the casing which is to be distorted inwards out of a high value/high strength material, in order to assist that section in withstanding high internal pressures and the effects of corrosion.
If it is not possible to achieve the necessary pressure resistance whilst allowing the necessary distortion for clamping to take place, then the thin walled tube may be externally reinforced to enable it to resist the hoop stresses arising when there is a high internal pressure.
The reinforcements may take the form of annular bands around the casing section, and these bands can provide the necessary thickness of material to allow a valve or valves to be fitted to the casing in the area where the casing wall is relatively thin.
The casing section where the compression unit is located may be readily separated from the rest of the casing, so that it can be replaced when necessary.
The compression unit preferably has a compression ring which is in contact with the outer surface of the large diameter casing and a compression collar which surrounds the ring and is axially movable relative to the ring. The ring may be split at one or more points around its circumference to assist assembly, and reduce radial stiffness.
The ring and the collar may each have one tapered annular surface. Alternatively, and preferably, the ring has two tapered surfaces, tapering in opposite axial directions, and the collar is split into two sections with opposite axial tapers and the means for producing the relative movement acts between the two sections of the collar to move the sections in opposite directions over the ring.
Preferably the ring has its region of greatest diameter between its two ends, and the two collar sections are drawn towards one another, for example by bolts through both sections, to compress the ring and thus to clamp the larger diameter casing onto the smaller diameter casing. Although a preferred method of drawing the two sections together is by use of a bolt, it should be understood that any suitable mechanism may be used bring the two sections together.
In addition to mechanical actuators, any suitable actuator may be used to create sufficient deflection in the outer casing. For instance, hydraulic pressure exerted on the exterior of the outer casing could be used or thermal energy could be used to create expansion and/or contraction of the outer casing thereby manipulating it""s internal diameter.
The clamping arrangement described here can be used, as described, to clamp a plain walled tube. In some circumstances however (particularly for small diameter casings) it may be expedient to provide a small hanger shoulder to take a part of the casing load and/or to locate the tubular casings in a desired axial position before applying a clamping arrangement as described here to clamp the casings in position.
According to a third aspect of the invention, there is provided a pre-installed clamping arrangement for clamping a subsequently installed tubular casing of a first diameter within a previously installed tubular casing of larger internal diameter, the arrangement comprising first and second compression rings having oppositely tapered external surfaces, an annular compression actuator having an internal tapered surface surrounding the first compression ring and an external tapered surface radially outside its internal tapered surface, and an annular component having two tapered surfaces, one of said surfaces mating with the second compression ring, and the other of said surfaces mating with the external tapered surface of the compression actuator, and means for moving the annular component axially relative to the compression rings and the compression actuator between a first position in which the tapered surfaces of the annular component exert no radial force on the compression rings or the compression actuator and a second position in which the tapered surfaces of the annular component exert sufficient radial force to distort the compression rings into the bore of the larger diameter casing, to grip the casing of smaller diameter.